The present invention relates to a radar and more particularly to radar systems having means for detecting coded information buried in radar echoes.
In the field of radar surveillance, IFF systems (identification friend or foe) have been proposed wherein friendly or cooperative targets are equipped with devices which artificially modulate radar reflections for the purpose of transmitting identification information in coded form from the target back to the radar receiver. Because an artificially modulated radar reflection from a friendly or cooperative target will appear in most respects as a common radar reflection, the code will be difficult to decipher. It has been generally known that an artificially modulated reflection is usually complicated by Doppler shifts of the natural and artificial signals caused by relative movements of the targets. Previously proposed methods of detection of the coded signal at the radar receiver by a correlation process required complicated and expensive Doppler filtering devices to extract the coded information. Therefore, those concerned with the improving of such devices have recognized the need for a more simple and effective detection circuit which is relatively immune to the Doppler effects. The present invention fulfills this need.